Most people in China have planted trees at some time or another, especially if they are officials doing their civic duty on national tree planting day. When the day was celebrated this month for the 20th successive year, an unbelievable figure was released for the number of trees planted since the first year in 1981.

The total, said the national greening commission (whose remit is to "greenify" China) amounted to some 35bn trees. Forest coverage had now reached 16.55% of China's land mass, as against only 12% two decades ago.

There were also encouraging figures for the length of windbreaks planted to prevent the desert from encroaching and to preserve soil and water in upland regions.

This should be a success story, yet everyone knows it is the reverse. Lakes and marshlands are drying up at an alarming rate in the watersheds of the Tibet-Qinghai plateau; Beijing is hit every year by more severe sandstorms; severe summer floods in central China are blamed on the denuded hills upstream. Indeed the commission's own report ended on a very different note from the one with which it began.

"Due to various factors," it concluded according to the China Daily's summary, "desertification, soil erosion, flooding, drought and other kinds of natural disasters still occur in China, and they are sometimes very harmful."

"The report shows that every year, an average of 2,460sq km [950sq miles] of vegetated land in China deteriorates into desert and another one million hectares of land suffers from serious land erosion."

So where have all those billions of trees gone?

As the environment become a more topical and politically acceptable subject, more people are posing the question. It was discussed at length during the National People's Congress in Beijing this month, particularly by specialists in the advisory body (the Chinese people's political consultative conference or CPPCC) which meets at the same time.

There is more than one answer, says one of the experts who attended the discussions.

"It may be true that they have planted those billions, but whether the trees will survive is another matter. They need care and attention over a long period of time, and they will only get it if there is proper management or an economic incentive to do so."

Windbreaks which surround commercial crops in inner Mongolia are much more likely to be properly maintained than those built in nonproductive areas to keep the encroaching desert at bay.

Hillsides where local farmers have a commercial stake in long-term forestry will also do better than those over which the community forages for firewood. Second, even where the new planting is properly maintained, it will take years - probably decades - before it can produce the same climatic benefits as the timber which it has replaced.

A third problem until recently has been a lack of forethought in planting. "They go in for monoculture and do not plant indigenous trees," says the expert. "A lot of eucalyptus is planted in the south but its leaves will not degrade."

Where timber is planted for periodic renewal, it is often felled on a wholesale basis so that vast areas are cut at the same time leaving the ground once again perilously denuded.

Anyone who travels in the west of China will soon encounter entire hillsides where the timber has been razed to the ground, or travel on roads where the tree cover on both sides has been felled for tens of kilometres.

China's richer communities may even pillage the tree resources of their rural interiors. Shanghai now boasts of its success in having transplanted 70,000 mature trees from its suburbs and from neighbouring provinces.

They include magnolias weighing ten tons and aged between 80 and a hundred years old, transported up to 700km from Anhui province.

Another 150,000 trees, says the city's gardening bureau, will be imported in the next two years.

Shanghai is another world in every sense from Tibet, where an experimental project has been launched to seed parts of the plateau by helicopter with grass and shrubs as well as trees. People are becoming more aware that in the headwaters of the river systems a mixed cover is more efficient.

Even if this works, the benefits should be set against exploitation of timber elsewhere in Tibet - a balance sheet which for political reasons is difficult to strike.

It is an uphill task in both senses of the phrase. There are, however, encouraging signs of a greater understanding on the need to plant and maintain trees on a long-term basis, says the CPPCC expert. Tree planting, after all, has deep roots in Chinese culture.

"Still planting trees at seventy!", wrote the 18th century poet Yuan Mei. "Yet mock not at my 'foolishness', dear neighbours. / It's true that mortals must die, / But fortunately no one can foretell when."